Glasses that are able to transmit light into the infrared region of the electromagnetic spectrum are well known in the art. U.S. Pat. No. 3,723,141 (W. H. Dumbaugh, Jr. et al.) provides a brief review of infrared transmitting Heavy Metal Oxide (HMO) glass compositions. One use for such glasses is as optical elements in commercial detection systems based on sensitivity to infrared radiation. Examples of such elements include windows, lenses, and telescopes used in the detection of thermal energy.
Because of atmospheric interferences which occur throughout the infrared region of the spectrum, optical engineers typically define two operating regimes which are differentiated on the basis of wavelength: the 3-5 micron regime and the 8-12 micron regime. The former region corresponds to the spectral radiant emittance of a relatively hot object such as engine exhaust. An example of an optical system operating in this region is the guidance system in a heat seeking missile. The latter region is employed by systems designed to detect objects at room temperature such as night vision devices.
Traditionally, glasses based on the oxides of silicon or germanium have been the materials of choice for use in the 3-5 micron range of the electromagnetic spectrum due to their relative ease of fabrication. However, these glasses have limited transmission in this region due to cutoffs that range from 4.5 to 5.5 microns. Consequently, significant degradation of the transmission at wavelengths as low as 4 microns is exhibited.
More exotic oxide glasses, such as those based on the oxides of aluminum or tellurium, have slightly better transmissions than the silicate or germanate glasses, but are hampered by poor chemical durabilities and processing complexities. The same can be said about the non-oxide glasses, such as the fluorides or non-oxide chalcogenides, which are known to have superior infrared transmitting abilities, but are hampered by the same shortcomings.
Stable, easily formed oxide glasses having the ability to transmit a high degree of light to wavelengths of 5 microns or more could be extremely useful in the applications described above, such as missile radomes, or in other optical components, such as space based lenses and mirrors. The glasses of the present invention are well suited for such uses.
Therefore, it is an object of this invention to provide a family of thermally stable, chemically durable, easily-formed alkali metal oxide glasses that exhibit good transmission in the 3-5 micron regime of the electromagnetic spectrum. It is a further object of this invention to show that said glasses exhibit good transmission to wavelengths of about 7 microns.